Metal oxide powders or their mixtures and their use in catalytic dehydrogenation of hydrocarbons

ABSTRACT

Metal oxide powders comprised of Cr(III) oxide, Ti(IV) oxide, V(V) oxide, or mixtures of these, or metal mixed oxides comprised of Cr(III) oxide and Ti(IV) oxide and V(V) oxide, or their mixtures. They have BET surfaces of 5-50 m 2  /g and mean particle diameters of 25-350 nm and are useful to increase conversion and selectivity in the manufacture of mono-olefins by catalytic dehydrogenation of saturated hydrocarbons. The metal oxide powders are produced from mixtures of the vaporized metal compounds chromyl chloride, titanium tetrachloride, and vanadyl chloride, in the presence of certain gases by laser pyrolysis.

FIELD OF THE INVENTION

The invention relates to metal oxide powders, their mixtures, metalmixed oxide powders, their mixtures, and use of these in catalyticdehydrogenation of branched or unbranched, saturated or unsaturatedhydrocarbons having 2-6 C atoms, or in catalytic dehydrogenation ofmixtures of such hydrocarbons, at a temperature of 300°-700° C. and apressure of 0.1-20 bar, possibly in the presence of hydrogen, steam,oxygen, or mixtures of hydrogen, steam, and/or oxygen.

BACKGROUND OF THE INVENTION

The catalytic dehydrogenation of hydrocarbons, particularly short chainhydrocarbons with 2-6 C atoms, is carried out in known fashion, bypassing the hydrocarbons adiabatically through fixed bed reactors, ineither pure form or diluted with an inert gas. A plurality of fixed bedreactors operated in parallel may be used. In general thedehydrogenation catalyst is comprised of aluminum oxide with additivessuch as, e.g., chromium oxide or other metal oxides or mixtures of same(see Ger. OSs Nos. 36 14 854 and 36 20 481, and U.S. Pat. Nos. 2,943,067and 2,419,997).

Such metal oxides are prepared, e.g., with aqueous ammonia solution.Often suitable solutions of metal salts are applied onto supportmaterials such as aluminum oxide.

About 8 years ago, various groups of researchers undertook the study ofceramic powders produced by a novel method, wherein suitable gaseousstarting compounds were reacted under CO₂ laser irradiation to formpulverulent solids.

The powders formed have the following properties: Small particlediameter, uniform shape, narrow particle size distribution, high purity,low degree of agglomeration, and high surface activity (see J. Am.Ceramic Soc., 65, 7:324 ff.).

In U.S. Pat. No. 4,343,687, among other things the production of metaloxide powders by irradiating suitable starting compounds with a pulsedIR laser of a frequency not absorbed by the reaction mixture isdescribed. The resulting products are said to be suitable for catalyticpurposes. According to this method, e.g., a Cr₂ O₃ powder is produced ona support, in a closed static cell, by a laser-induced reaction betweenchromyl chloride (CrO₂ Cl₂) as the oxidant and hydrogen as the reactionpartner, wherewith the reaction is initiated by at least one laser pulseof 2 J/cm².

SUMMARY OF THE INVENTION

An underlying object of the present invention is to devise means ofproducing new metal oxide powders comprised of Cr(III) oxide, Ti(IV)oxide, V(V) oxide, or mixtures of these, and new metal mixed oxidescomprised of Cr(III) oxide and Ti(IV) oxide, Cr(III) oxide and V(V)oxide, or Ti(IV) oxide and V(V) oxide, or their mixtures, which haveadvantageous properties. A second underlying problem is to increase theconversion and selectivity of the manufacture of mono-olefins by thecatalytic dehydrogenation of saturated hydrocarbons, with the aid of theabovementioned compounds or mixtures of said compounds.

These problems are solved by the inventive metal oxide powders or theirmixtures and by the use of the inventive powders as dehydrogenationcatalysts.

The claimed metal oxide powders of the invention include novel metalmixed oxide powders comprised of Cr(III) oxide and Ti(IV) oxide, Cr(III)oxide and V(V) oxide, or Ti(IV) oxide and V(V) oxide, or their mixtures.These novel metal mixed oxide powders are distinguished from simplemixtures of the commercially avialable individual oxides, and are evendistinguished from mixtures of individual metal oxide powders whichindividual metal oxide powders have been prepared according to theinvention, namely, mixtures of Cr(III) oxide, Ti(IV) oxide, and V(V)oxide individually, having a BET surface of 5-50 m² /g and mean particlediameter 25-350 nm. The novel metal oxide powders are prepared fromvaporized metal compounds, namely chromyl chloride, titaniumtetrachloride, and vanadyl chloride; and the novel metal mixed oxidepowders are prepared from mixtures of said vaporized metal compounds.

The metal oxide powders and metal mixed oxide powders are prepared fromsaid vaporized volatile metal compounds in the presence of a noble gasor hydrogen, nitrogen, sulfur hexafluoride, or oxygen, or some mixtureof these gases, with the method of preparation being continuous laserpyrolysis at 10-1000 mbar.

The invention further includes the use of the above-described metalmixed oxide powders or their mixtures, and the use of a representativeof the group comprised of Cr(III) oxide, Ti(IV) oxide, and V(V) oxide,or a mixture of these, for catalytic dehydrogenation of branched orunbranched, saturated or unsaturated hydrocarbons with 2-6 C atoms, ormixtures of such hydrocarbons, at a temperature of 300°-700° C. and apressure of 0.1-20 bar.

Preferably the catalytic dehydrogenation is carried out in the presenceof hydrogen, steam, oxygen, or a mixture of these.

It is also preferred to use the inventive metal mixed oxide powders ortheir mixtures, and the inventive metal oxide powders or their mixtures,for catalytic dehydrogenation of the aforesaid hydrocarbons, wherein thedehydrogenation is carried out in the presence of aluminum hydroxidepowder in the amount of up to 90 wt. %, wherein the materials are usedin the form of molded bodies produced with a binding agent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an apparatus suitable for the preparation of the metaloxide powders.

FIG. 2 is an IR spectrum of Cr₂ O₃ of the invention and of commercialCr₂ O₃.

FIG. 3 is an IR spectrum of TiO₂ /Cr₂ O₃ according to the invention andof commercial TiO₂ and Cr₂ O₃.

FIG. 4 is an X-ray diffraction of TiO₂ /Cr₂ O₃ according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The apparatus (FIG. 1) employed for decomposing the gaseous startingcompounds and separating out the resulting solid products, such asTi(IV) oxide (TiO₂), V(V) oxide (V₂ O₅), and Cr(III) oxide (Cr₂ O₃), andmetal mixed oxide powders, is comprised of glass material. The laserbeam enters the cell 2 through NaCl windows 1, passes out through suchwindows 1, and arrives finally at a detector 3 having a power-measuringdevice 4. The CO₂ laser employed has an emission spectrum comprisingmore than 200 lines.

The CO₂ continuous laser has an efficiency of 15%, and delivers 75-85 Wat the 984 cm⁻¹ line, c. 100 W at 945 cm⁻¹, and c. 75 W at 1042 cm⁻¹. At80 W total power, the power density without focusing is c. 3,000 W/cm²,and is c. 30,000 W/cm² with focusing.

The gaseous products flow out through a slit-shaped nozzle 5 or a roundnozzle, in a direction perpendicular to the laser beam. Prior to thereaction, the starting compounds are cooled in a reservoir flask 6 bydry ice and methanol, to prevent premature evaporation. During thereaction, the flask must be gently heated. The feed tube 7 to the nozzleis heated with a heating wire 8 to prevent condensation of the startingcompound.

The diameter of the laser beam when unfocused is c. 2 mm; focused0.5-0.7 mm.

A gas stream comprised of a noble gas, hydrogen, nitrogen, sulfurhexafluoride, or oxygen, or a mixture of these, is passed around thenozzle in order to ensure laminar flow of the educts and products and/orto influence the reaction. The supplementary gas is introduced at theNaCl windows via pipes 9 and 10, to prevent soiling of the windows bythe powder produced in the laser pyrolysis. The total pressure,registered on a manometer 11, is maintained typically at 50 mbar byregulating the pumping rate. A reaction can be achieved with totalpressure in the range 10-1000 mbar.

The powder is collected on a membrane filter or in a fiber filter 12.Condensable gases and unconverted material are condensed out in coldtraps 13 downstream of the filter, and are either subsequently discardedor are processed for reuse.

The laser beam may be focused on a point above the nozzle, using a ZnSefocusing lens with focal length f=30 cm, or a NaCl focusing lens withfocal length f=20 cm. To increase the yield, a multipass cell may beused, wherein the flow of the starting compound is passed a number oftimes perpendicularly to the laser beam. The same purpose is served by acell wherein the laser beam passes four times through the same regionabove the nozzle, in the horizontal plane, by means of four entranceopenings arranged in the form of a cross.

The invention will be described in more detail hereinbelow, and tests offormation of Cr₂ O₃ powder, TiO₂ powder, and V₂ O₅ powder will bedescribed.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The decomposition of titanium tetrachloride (TiCl₄) is carried out in anargon-oxygen stream, in the above-described apparatus. TiCl₄ absorbs at984 cm⁻¹. The absorption of TiCl₄ at this line and elsewhere in theemission range of the CO₂ laser is low. If the reservoir flask is heatedstrongly, a reaction is attained, with powder formation and a brightpink-violet flame. The yield is a few percent.

X-ray diffractometry shows that the reaction product is comprised of thetwo titanium dioxide forms, rutile and anatase.

Transmission electron micrographs (TEM) show that the TiO₂ particlesproduced have mean particle size 25 nm, and a very narrow particle sizedistribution.

The decomposition of vanadyl chloride (VOCl₃) is also carried out in theabove-described apparatus. The exposure is effected at a P-line of thelaser at 1042 cm⁻¹. VOCl₃ absorbes the CO₂ laser radiation better thanTiCl₄, but still to a much lesser degree than CrO₂ Cl₂. In anargon-oxygen stream it is possible to achieve a continuous reaction withformation of powder and a pale yellow flame. The yield ofyellowish-brown V₂ O₅ is only a few percent, however. Diffractometryshows that the V₂ O₅ produced is X-ray amorphous. It is not possible toachieve decompositions on other lines.

Heretofore attempts to achieve a continuous reaction in anargon-hydrogen stream have not succeeded. A weak reaction is produced,with only a small, pale flame, yielding a small amount of black V(III)oxide, at a yield of <1%.

CrO₂ Cl₂ has proved to be the most suitable readily vaporizable chromiumcompound for producing catalytic Cr₂ O₃. The thermal decomposition ofCrO₂ Cl₂ by conventional methods at temperatures up to 380° C. leadsprincipally to Cr(IV) oxide. Increasing the temperature to above 400° C.leads to a stabile Cr(III) oxide, with loss of oxygen.

The CO₂ laser line at 984 cm⁻¹ is the most suitable for decomposing CrO₂Cl₂, but pyrolysis is also possible on neighboring lines.

If sulfur hexafluoride (SF₆) is used as a carrier gas, the irradiationcan be carried out to form Cr₂ O₃ even at the 945 cm⁻¹ absorption bandof SF₆. However, if a carrier gas other than SF₆ is used, CrO₂ Cl₂ doesnot decompose under irradiation by CO₂ laser light of wavelength 945cm⁻¹.

The CrO₂ Cl₂ vapor absorbs 2-8 W of the laser power applied.

The CrO₂ Cl₂ partial pressure is 2-10 mbar.

Depending on the system conditions, 1.5-12 g Cr₂ O₃ per hr can besynthesized.

The flame was orange when decomposing CrO₂ Cl₂ with a noble gas,nitrogen, oxygen, or SF₆ as the carrier gas. Depending on conditions,flame size was 3-20 mm and began 2-4 mm above the nozzle.

In the tests with pure hydrogen, the flame burned only very irregularly,with a luminous bright yellow color, and began right at the nozzleopening or even inside the nozzle. Accordingly, it was difficult toachieve a long continuous run with powder production.

It was possible to achieve smooth operation for an extended time if thehydrogen was diluted with argon in a volumetric ratio of 1:8.

In studies with hydrogen and argon, it was possible to convert nearly100% of the chromyl chloride. With argon alone, or nitrogen or oxygen,maximum conversions were only 45-70%. In studies with SF₆ as a carriergas and absorber of the laser radiation, only 30% of the CrO₂ Cl₂ wasconverted. With helium as a carrier gas, the yield was reduced to c.20%.

Radiation from a continuous laser of, e.g., 100-1000 W is continuouslyabsorbed directly in the gaseous metal compounds, e.g. CrO₂ Cl₂, or inan energy transferring gas, e.g. SF₆, whereby the chemical reaction toform the novel oxides or oxide mixtures is brought about.

Because the chemical reaction is in fact maintained by the laserradiation, the course of the reaction can be controlled to produce afiner or coarser powder, or even a powder with more or fewer faultlocations in the crystal lattice of the metal oxides or metal mixedoxides.

The BET method is used to determine the specific surface of the Cr₂ O₃powder product. Commercial Cr₂ O₃ has a specific surface of 2-3 sq m/g.All powders produced according to the described method have specificsurfaces 5-20 times greater than this.

The Cr₂ O₃ product was characterized with the aid of the Guinier X-raymethod. The lines on the film strips indicated the same elementary cellwith the same dimensions and with the same line sensitivity ratios ascommercial Cr₂ O₃. Samples from experiments with hydrogen and/or argonhad particle sizes clearly below 100 nm based on TEM examination.Accordingly, lines which were broadened and blurred were observed in theGuinier spectrum.

X-ray diffractometry and X-ray fluorescence analysis showed that thepowders produced are comprised of very pure Cr₂ O₃. The only appreciableimpurity detected was the element chlorine, present at the lowconcentration of 400-1500 ppm. The samples with SF₆ as a carrier gas andabsorber gas contained only traces of sulfur (20 ppm).

IR spectroscopy also showed that the Cr₂ O₃ product produced accordingto the invention was very pure (see FIG. 2). (KEY to FIG. 2:--=Cr₂ O₃according to the invention;----=commercial Cr₂ O₃.)

Studies by scanning electron microscope (SEM) and TEM showed a narrowparticle size distribution. The mean particle size of the various Cr₂ O₃powders was between 50 and 350 nm, whereas that of commercial Cr₂ O₃ wasfound to be 800 nm.

Electron spin resonance (ESR) showed that all the inventivelysynthesized powders have different magnetic behavior than commercial Cr₂O₃.

The inventive mixed oxides are prepared as follows: The vaporizedstarting compounds are irradiated with a continuous CO₂ laser ofvariable frequency, whereby either the starting compound absorbs thelaser energy or SF₆ serves as an absorbing auxiliary gas. The conversionmay be carried out in a continuous operation comprised of a plurality ofstages. The gases generated are continuously pumped off, and the powderproduced is captured in filters.

In studies on the production of mixed oxides or mixtures of oxides asingle reservoir flask was employed which was divided into two chambers.Each chamber was capable of being individually heated, in order toadjust the partial pressure of the starting compound in it to thedesired value. Thereby the mixing ratio of the gases could be freelyadjusted. During the reaction, the reservoir flask must be gentlyheated. The two vaporized compounds then flow out through a commonnozzle and through the laser beam.

To produce a mixed Cr₂ O₃ /TiO₂ oxide by this method, CrO₂ Cl₂ and TiCl₄are simultaneously vaporized, using a noble gas as a carrier, and areconverted in the laser beam. Other carrier gases such as, e.g.,nitrogen, oxygen, or steam, may also be used. The CrO₂ Cl₂ supplies theoxygen for forming the TiO₂.

The irradiation is carried out on the common 984 cm⁻¹ absorption line ofCrO₂ Cl₂ and TiCl₄. It is also possible to carry out pyrolysis withneighboring laser lines.

Under these conditions it is possible to achieve a continuous reactionwith powder production and with a flame which corresponds to thatobtained in the decomposition of pure CrO₂ Cl₂. Reaction is not possibleon other lines of the CO₂ laser.

As a result of the arbitrarily adjustable gas composition, oxides with awide range of mixture ratios can be produced. With a mixture ratio of,e.g., 3:1 for CrO₂ Cl₂ : TiCl₄, a mixed oxide is obtained in an overallyield of c. 80%.

X-ray fluorescence analysis indicates Cr and Ti as chief constituents ofthe mixed oxide.

The IR spectrum (FIG. 3) shows characteristic bands at 1200 to 300 cm⁻¹,and clearly shows that the novel mixed oxide of Cr₂ O₃ and TiO₂ has adifferent structure than that of commercial Cr₂ O₃ and commercial TiO₂,as well as a different structure than that of the mixture of inventiveCr₂ O₃ and inventive TiO₂. (KEY to FIG. 3:--=mixture of TiO₂ and Cr₂ O₃according to the invention; -.-.-=commercial TiO₂ ;---=commercial Cr₂O₃.)

X-ray diffractometry (FIG. 4) clearly shows widened bands associatedwith Cr₂ O₃. No bands corresponding to any TiO₂ modification appear.This indicates that Ti atoms have been substituted for Cr atoms in theCr₂ O₃ lattice. (KEY to FIG. 4: (a) Intensity, cps; (b) Angle ofdiffraction, degrees.)

To produce mixtures of Cr₂ O₃ and TiCl₄, one converts CrO₂ Cl₂ and VOCl₃simultaneously in the laser beam, with a noble gas again being used as acarrier gas.

According to the invention, the conversion and yield are increased inthe dehydrogenation of saturated hydrocarbons having a small number of Catoms, when one employs metal mixed oxide powders produced by laserpyrolysis, or mixtures of such mixed oxide powders, or metal oxidepowders thus produced, or mixtures of such metal oxide powders. This hasbeen confirmed by experiment.

In the example set forth hereinbelow, molded bodies produced frominventively manufactured Cr₂ O₃ were employed to increase yields andconversions in the dehydrogenation of isobutane, in comparison to theresults when commercially marketed Cr₂ O₃ was employed as the catalystsubstance.

EXAMPLES

(a) Isobutane was passed at 593° C. and 300 mbar through a tubularreactor in which a fixed bed of catalyst was disposed. The reaction wascarried out for 10 min in each case. The catalyst loading was 2 gisobutane per gram catalyst per hr.

The catalyst bed was comprised of a prescribed amount of catalystpellets each having dimensions c. 3×4 mm. The pellets were dried forseveral hours at 150° C., followed by calcination 3 hr at 550° C.

Following the reaction, nitrogen purging was carried out. The catalystwas then regenerated by roasting in a nitrogen-oxygen mixture.

The reaction products were analyzed by GC at the outlet of the reactor.The carbon which separated out on the catalyst was determined as CO₂ inan IR analyzer by roasting the catalyst in air.

EXPERIMENT aA Comparison Example

Conventionally produced commercial Cr₂ O₃ :

Feed: Isobutane 99.9 wt. %.

Results of GC and IR analyses at the reactor outlet:

    ______________________________________                                        GC Analysis                                                                   Methane             0.09   wt. %                                              Propane             0.01   wt. %                                              Propene             0.20   wt. %                                              Isobutane           99.09  wt. %                                              n-Butane            0.01   wt. %                                              Isobutene           0.57   wt. %                                              H.sub.2             0.02   wt. %                                              IR Analysis                                                                   Carbon              0      wt. %                                              Conversion:         1      mol %                                              Selectivity:        73     mol %                                              Yield:              1      mol %                                              ______________________________________                                    

EXPERIMENT aB

Cr₂ O₃ produced by laser pyrolysis in a hydrogen-and-noble-gas mixtureemployed as a carrier gas:

Feed: Isobutane 99.9 wt. %.

Results of GC and IR analyses at the reactor outlet:

    ______________________________________                                        GC Analysis                                                                   Methane                0.92   wt. %                                           Ethane                 0.28   wt. %                                           Ethene                 0.16   wt. %                                           Propane                0.41   wt. %                                           Propene                1.11   wt. %                                           Isobutane              58.23  wt. %                                           n-Butane               0.09   wt. %                                           1-Butene               0.15   wt. %                                           Isobutene              33.84  wt. %                                           trans-2-Butene         0.28   wt. %                                           cis-2-Butene           0.19   wt. %                                           1,3-Butadiene          0.14   wt. %                                           Hydrocarbons with > 5 C atoms                                                                        0.12   wt. %                                           H.sub.2                1.88   wt. %                                           CO                     0.7    wt. %                                           IR Analysis                                                                   Carbon                 1.53   wt. %                                           Conversion:            42     mol %                                           Selectivity:           84     mol %                                           Yield:                 35     mol %                                           ______________________________________                                    

(b) Isobutane was dehydrogenated in the same apparatus as in (a), andagain at 593° C. and 300 mbar, with reaction time 10 min. However, herethe catalyst loading was only 0.5 g isobutane per gram catalyst perhour.

The catalyst bed was comprised of pellets produced as follows:

15 parts by wt. chromium oxide powder was mixed thoroughly with 85 partsby wt. aluminum hydroxide. 2.5 parts by wt. sodium silicate solution wasadded to 97.5 parts by wt. of this mixture. Additional water was addedto form a brushable paste.

Molded bodies (pellets) 3×4 mm each were produced from this paste. Thesewere dried several hours at 150° C., and were then calcined 3 hr at 550°C.

The regeneration of the catalyst and the analyses of the reactionproducts were performed analogously to (a), supra.

EXPERIMENT bA Comparison Example

Conventionally produced commercial Cr₂ O₃ :

Feed: Isobutane 99.9 wt. %.

Results of GC and IR analyses at the reactor outlet:

    ______________________________________                                        GC Analysis                                                                   Ethene              0.18   wt. %                                              Methane             2.83   wt. %                                              Propane             0.11   wt. %                                              Propene             1.87   wt. %                                              Isobutane           84.65  wt. %                                              1-Butene            0.11   wt. %                                              Isobutene           7.4    wt. %                                              trans-2-Butene      0.08   wt. %                                              cis-2-Butene        0.06   wt. %                                              H.sub.2             1.82   wt. %                                              CO                  0.36   wt. %                                              CO.sub.2            0.1    wt. %                                              1,3-Butadiene       0.04   wt. %                                              IR Analysis                                                                   Carbon              0.44   wt. %                                              Conversion:         15     [mol %]                                            Selectivity:        50     [mol %]                                            Yield:              8      [mol %]                                            ______________________________________                                    

EXPERIMENT bB

Cr₂ O₃ produced by laser pyrolysis in argon employed as a carrier gas:

Feed: Isobutane 99.9 wt. %.

Results of GC and IR analyses at the reactor outlet:

    ______________________________________                                        GC Analysis                                                                   Methane                1.27   wt. %                                           Ethane                 0.28   wt. %                                           Ethene                 0.15   wt. %                                           Propane                0.5    wt. %                                           Propene                1.95   wt. %                                           Isobutane              53.89  wt. %                                           n-Butane               0.11   wt. %                                           1-Butene               0.21   wt. %                                           Isobutene              36.50  wt. %                                           trans-2-Butene         0.29   wt. %                                           cis-2-Butene           0.20   wt. %                                           1,3-Butadiene          0.16   wt. %                                           Hydrocarbons with > 5 C atoms                                                                        0.03   wt. %                                           H.sub.2                2.74   wt. %                                           CO                     0.3    wt. %                                           IR Analysis                                                                   Carbon                 1.93   wt. %                                           Conversion:            46     [mol %]                                         Selectivity:           82     [mol %]                                         Yield:                 38     [mol %]                                         ______________________________________                                    

(c) Isobutane was dehydrogenated in the same apparatus as in (a), andagain at 300 mbar, with reaction time 10 min, and catalyst loading 1 gisobutane per gram catalyst per hour, as in (a). However, here thetemperature was 566° C.

The catalyst bed was comprised of pellets produced as follows:

The starting materials Cr₂ O₃ (61.5 wt. %), melamine (0.5 wt. %) as apore-former, and boehmite (3 wt. %) ("Pural SB", provided by the firmCondea) as a binder, wore intermixed thoroughly in the dry state in theweight ratios stated below. 1M formic acid was added to this mixtureunder constant stirring, with addition continuing until a kneadable masswas produced from which the initially moist catalyst pellets could besuitably prepared.

The molded bodies (pellets) were dried at 50° C. in a nitrogen streamfor 2-5 hr, followed by calcining at 550° C. for 5 hr. The result waspellets comprised of Cr₂ O₃ in the amount of 95 wt. % and boehmite inthe amount of 5 wt. %.

The regeneration of the catalyst and the analyses of the reactionproducts were performed analogously to (a), supra.

EXPERIMENT cA Comparison Example

Conventionally produced commercial Cr₂ O₃ :

Feed: Isobutane 99.9 wt. %.

Results of GC and IR analyses at the reactor outlet:

    ______________________________________                                        GC Analysis                                                                   Methane              0.22   wt. %                                             Ethane               0.02   wt. %                                             Ethene               0.02   wt. %                                             Propane              0.08   wt. %                                             Propene              0.35   wt. %                                             Isobutane            77.73  wt. %                                             n-Butane             0.02   wt. %                                             Isobutene            19.97  wt. %                                             trans-2-Butene       0.07   wt. %                                             cis-2-Butene         0.04   wt. %                                             H.sub.2              1.22   wt. %                                             CO                   0.03   wt. %                                             IR Analysis                                                                   Carbon               0.25   wt. %                                             Conversion:          22     mol %                                             Selectivity:         93     mol %                                             Yield:               20     mol %                                             ______________________________________                                    

EXPERIMENT cB

Cr₂ O₃ produced by laser pyrolysis in nitrogen employed as a carriergas:

Feed: Isobutane 99.9 wt. %.

Results of GC and IR analyses at the reactor outlet:

    ______________________________________                                        GC Analysis                                                                   Methane                0.88   wt. %                                           Ethane                 0.22   wt. %                                           Ethene                 0.12   wt. %                                           Propane                0.38   wt. %                                           Propene                0.87   wt. %                                           Isobutane              60.68  wt. %                                           n-Butane               0.28   wt. %                                           1-Butene               0.43   wt. %                                           Isobutene              31.38  wt. %                                           trans-2-Butene         0.58   wt. %                                           cis-2-Butene           0.40   wt. %                                           1,3-Butadiene          0.19   wt. %                                           Hydrocarbons with > 5 C atoms                                                                        0.04   wt. %                                           H.sub.2                1.90   wt. %                                           CO                     0.28   wt. %                                           IR Analysis                                                                   Carbon                 1.45   wt. %                                           Conversion:            39     mol %                                           Selectivity:           83     mol %                                           Yield:                 32     mol %                                           ______________________________________                                    

EXPERIMENT cC

Cr₂ O₃ produced by laser pyrolysis in oxygen employed as a carrier gas:

Feed: Isobutane 99.9 wt. %.

Results of GC and IR analyses at the reactor outlet:

    ______________________________________                                        GC Analysis                                                                   Methane                1.91   wt. %                                           Ethane                 0.56   wt. %                                           Ethene                 0.15   wt. %                                           Propane                0.93   wt. %                                           Propene                1.17   wt. %                                           Isobutane              46.51  wt. %                                           n-Butane               0.30   wt. %                                           1-Butene               0.30   wt. %                                           Isobutene              39.37  wt. %                                           trans-2-Butene         0.40   wt. %                                           cis-2-Butene           0.27   wt. %                                           1,3-Butadiene          0.10   wt. %                                           Hydrocarbons with > 5 C atoms                                                                        0.03   wt. %                                           H.sub.2                3.15   wt. %                                           CO.sub.2               0.04   wt. %                                           CO                     1.08   wt. %                                           IR Analysis                                                                   Carbon                 4.02   wt. %                                           Conversion:            53     mol %                                           Selectivity:           76     mol %                                           Yield:                 40     mol %                                           ______________________________________                                    

EXPERIMENT cD

Cr₂ O₃ produced by laser pyrolysis in oxygen employed as a carrier gas.Pellets as in Experiment cC, but additionally impregnated with KHCO₃solution:

Feed: Isobutane 99.9 wt. %.

Results of GC and IR analyses at the reactor outlet:

    ______________________________________                                        GC Analysis                                                                   Methane                1.20   wt. %                                           Ethane                 0.79   wt. %                                           Ethene                 0.08   wt. %                                           Propane                0.67   wt. %                                           Propene                0.96   wt. %                                           Isobutane              46.44  wt. %                                           n-Butane               0.24   wt. %                                           Isobutene              44.30  wt. %                                           trans-2-Butene         0.30   wt. %                                           cis-2-Butene           0.21   wt. %                                           1,3-Butadiene          0.10   wt. %                                           Hydrocarbons with > 5 C atoms                                                                        0.03   wt. %                                           H.sub.2                2.56   wt. %                                           CO.sub.2               0.14   wt. %                                           CO                     0.48   wt. %                                           IR Analysis                                                                   Carbon                 2.05   wt. %                                           Conversion:            53     mol %                                           Selectivity:           86     mol %                                           Yield:                 46     mol %                                           ______________________________________                                    

We claim:
 1. A metal oxide powder comprising Cr (III) oxide, Ti (IV)oxide, V (V) oxide, or mixtures thereof, having a BET surface area of5-50 m² /g and a mean particle diameter of 25-350 nm, having beenprepared by continuous laser pyrolysis at a pressure of 10-1000 mbar ofvaporized chromyl chloride, titanium tetrachloride, vanadyl chloride, ormixtures thereof, in the presence of a gas of hydrogen, nitrogen, sulfurhexafluoride, or oxygen, or a mixture of any of these gases with a noblegas, or in the presence of a noble gas.
 2. The metal oxide powderaccording to claim 1, comprising a mixture of at least two of Cr (III)oxide, Ti (IV) oxide and V (V) oxide.
 3. The metal oxide powderaccording to claim 2, wherein said mixture comprises Cr (III) oxide andTi (V) oxide.
 4. The metal oxide powder according to claim 2, comprisingCr (III) oxide and V (V) oxide.
 5. The metal oxide powder according toclaim 2, comprising Ti (IV) oxide and V (V) oxide.
 6. The metal oxidepowder according to claim 2, comprising Cr (III) oxide, Ti (V) oxide andV (V) oxide.
 7. A molded body comprising a binder, aluminum hydroxide inan amount of up to 90 wt. %, and a metal oxide powder comprising Cr(III) oxide, Ti (IV) oxide, V (V) oxide, or mixtures thereof, having aBET surface area of 5-50 m² /g and a mean particle diameter of 25-350nm, having been prepared by continuous laser pyrolysis at a pressure of10-1000 mbar of vaporized chromyl chloride, titanium tetrachloride,vanadyl chloride, or mixtures thereof, in the presence of a gas ofhydrogen, nitrogen, sulfur hexafluoride, or oxygen, or a mixture of anyof these gases with a noble gas, or in the presence of a noble gas. 8.The molded body according to claim 7, wherein said metal oxide powdercomprises a mixture of at least two of Cr (III) oxide, Ti (IV) oxide andV (V) oxide.
 9. The molded body according to claim 7, wherein said metaloxide powder comprises Cr (III) oxide and Ti (V) oxide.
 10. The moldedbody according to claim 7, wherein said metal oxide powder comprises Cr(III) oxide and V (V) oxide.
 11. The molded body according to claim 7,wherein said metal oxide powder comprises Cr (III) oxide, Ti (IV) oxideand V (V) oxide.